1. Field of the Invention
The present invention relates to an invisible antenna for radio communication, which receives a VHF band, UHF band, etc., in more particularly, to an invisible antenna with a reduced visibility of an antenna device.
2. Description of the Related Art
Conventionally, when considering a half wavelength dipole antenna as antenna device for transmitting and receiving a VHF band (30 to 300 MHz), UHF band (300 MHz to 3 GHz), etc., an antenna device 30 comprising a pair of conductor plates 31, 31 and a feeder part 32 connected to the conductor plates 31, 31 may be provided as shown in FIG. 1.
Herein, the conductor plate 31 can be composed of a pipe material or wire rod. A total length L of the conductor plates is various, however, as for a most theoretical length, the length L is the ½ wavelength. For example, the length L becomes about 300 mm (L=300 mm) for a 500 MHz band, since the wavelength is 600 mm. For this case, a width W of the conductor plate is generally more than several millimeters for the practical dimension.
In addition, FIG. 2 shows another type of a conventional antenna device 30 in which a passive element 33 is disposed with a predetermined distance from conductor plates 31, 31 to adjust directional characteristics. FIG. 3 shows an antenna device 30, in which a pair of triangular conductor plates 31a, 31a are positioned in symmetry to provide a bow tie configuration, so as to broaden a bandwidth of a resonance frequency. FIG. 4 shows a conventional antenna device 30, in which a pair of fan-shaped conductor plates 31b, 31b are positioned in symmetry to provide a bow tie configuration, so as to broaden a bandwidth of the resonance frequency.
However, in the conventional antenna device 30 for example shown in FIG. 1, the width W of the conductor plate 31 is several millimeters and the length L is about 300 mm for the 500 MHz band. Since the width N and the length L are large, the antenna device 30 is visible by human naked eyes. For example, if an installation site of the antenna device 30 is a perimeter of a television receiver or inside of a car, the existence of the antenna device may be an issue in a total design matching.
In addition, film-shaped antennae have been commercialized. However, when the film-shaped antenna is stuck on a glass window of a house or car, the existence of the antenna device may become an issue in the total design matching. When the antenna device occupies a large area, it may become one of visual field blockage factors. Conventional film-shaped antennae are disclosed in Japanese Patent Laid-Open (Kokai) Nos. 2000-174529 (JP-A-2000-174529), 11-145717 (JP-A-11-145717), and 8-242114 (JP-A-8-242114).
For solving the above problems, as shown in FIG. 5, it is necessary to make a width of linear conductors 21, 21 extremely small, such that the linear conductors 21, 21 are not in a visible state. However, such a configuration is accompanied with an increase in a conductor resistance, thereby occurring a loss in electric wave transmission and reception characteristics, which is an important function of the antenna device.
In other words, it is necessary for the length L of the linear conductors 21, 21 to be about ½ wavelength to tune the resonance frequency in the dipole antenna. However, when the width W of the linear conductor 21 is made small, the conductor resistance is increased, so that the conductor resistance becomes dominant in an input impedance of the antenna. As a result, there is a disadvantage in that an impedance matching with a feeder part 22 becomes impossible, thereby deteriorating the antenna characteristics.
Accordingly, it is an object of the invention to provide an invisible antenna with excellent antenna characteristics, which cannot be recognized visually by the human naked eyes.
According to a first feature of the invention, an invisible antenna, comprises:
a transparent insulating layer; and
a plurality of conductors disposed on and/or in the transparent insulating layer for radiating or receiving electric wave, the conductor being invisible by human eyes.
Further, the conductor may be a linear conductor.
It is preferable that a diameter of the conductor is 0.1 mm or less. It is more preferable that a diameter of the conductor is 0.08 mm or less.
Still further, the invisible antenna may further comprise a feeding line connected to a pair of the conductors, wherein the conductors are disposed in parallel with each other.
In addition, the invisible antenna may further comprise a feeding line connected to a pair of the conductors, wherein each of the conductors is provided with an angle different with each other to a reference line.
An angle between adjacent ones of the conductors may be different with each other. The angle between adjacent ones of the conductors may be equal to each other. Each of the conductors may be provided with a length different with each other.
Further, the conductors may be crisscrossed with each other.
Still further, the conductors may be formed on and/or in the insulating layer by a mechanical process. The conductors may be formed on and/or in the insulating layer by a chemical process.
Furthermore, it is preferable that a projection width of the conductor on a plane observed by human eyes is 0.1 mm or less, and a pitch between adjacent ones of the conductors on the plane is more than ten times of the diameter of the conductor or the projection width of the conductor at the narrowest.
The projection width of the conductor may be 0.08 mm or less.
According to an invisible antenna of the present invention, since extremely thin linear conductors are disposed planarly with a large pitch, a visual recognition of the antenna device by human naked eyes becomes almost impossible, so that the installation condition of the antenna device will not become an issue in the total design matching. In addition, while the antenna device may be provided in various shapes in accordance with its application, the invisible antenna of the present invention can be freely formed in any shape, since the visibility of the antenna device becomes almost none.